Big Boar Products claim to make a wide tire kit for the 1984-85 fxst softails here is a response for the email i sent to them for my 1985 4sp fxst softail (Yes, we have them for your bike,$1995.00, includes swingarm, offset kit, tire and rear fender, or $2295.00with 60 spoke wheel)
If this is true it would be great,wonder if they know what their talking about.the softail frame was changed in 1986 and made wider and I couldn't find anyone who made a kit for these two years.